This invention relates to printed circuit boards suitable for mounting electronic parts such as integrated circuit parts, having lead terminals provided at very small pitch intervals.
Generally, the surface of a printed circuit board on which surface a conductor pattern of copper or like conductor is formed is dipped through a fused solder bath or in a jet type solder bath after the electronic parts are assembled on the printed circuit board, and solder is attached to the conductive pattern surface. Thus, the leads of the electronic parts are electrically and mechanically connected to the conductor pattern. The printed circuit board is used after making these connections.
In the conductor pattern as mentioned above a layer for the resisting solder; that is the so-called solder resist layer, is formed so that solder may be attached to only fixed portions of the conductor pattern.
In the meantime, there is a trend for the gap between adjacent lands of the conducting pattern to be reduced with a tendency toward reduction in the size of electric apparatus and the use of many semiconductor integrated circuit parts, which use has been made possible with the progress of techniques concerning electronic parts; in fact it is often the case in these days that interland gaps less than the minimum gap of 0.5 millimeter prescribed in the Japanese Industrial Standard (JIS C-5010) are required.
For example, when soldering lead terminals arranged in a row such as those in a semiconductor integrated circuit component arranged at a pitch of 2.5 millimeters to the circular portion of conductor lands arranged in correspondence with the row, solder resist layers are printed with space between the layers of 0.5 millimeter or less.